Nucleic acids representing the genomic RNA of oncornaviruses and virus-associated, ribosome-like RNA were isolated from purified virions and visualized in the electron microscope by a modified Kleinschmidt-technique. Their lengths were statistically analyzed and secondary structure maps were constructed. The results indicate 1) the presence of a heterogeneous population with four distinct size groups in addition to the viral genome of S35, and 2) complete agreement of molecular weight and structure of viral ribosome-like RNA with cellular ribosomal RNA. A time study was undertaken combining morphological methods with tracer studies on the entry of herpes type viruses into cells, and subsequent early intracellular events have been followed by electron microscopy.